Two principal factors which limit the efficiency of infrared emitting diodes are the absorption of the generative radiation of the semiconductor material before emission and, the total internal reflection at the semiconductor-air interface. In GaAs flat devices, the absorption can be minimized by the use of thin transparent layers of high bandgap energy GaAlAs. The high refractive index of GaAs (n.sub.s =3.6) compared to that of air (n.sub.a =1.0) results in a large refraction of rays at the semiconductor-air interface for rays which are not normal to the interface. Total internal reflection of rays occurs for angles greater than the critical angle (O.sub.s).sub.c defined by sin (.theta..sub.s).sub.c =n.sub.a /n.sub.s. Thus, only rays within a cone with a half angle of 16.1 degrees can be emitted through the top of the flat emitter. This corresponds to 2 percent of the total generated radiation, assuming no rays are reflected from the backside of the emitter. Shaped emitters can be used to eliminate total internal reflections. For a hemispherical emitter with a small junction diameter, all rays are incident approximately normal to the semiconductor-air interface and total internal reflection losses are eliminated. Ideally, a factor of 13 increase in radiant intensity (W/sr) can be achieved by use of a hemispherical emitter instead of a flat emitter. A disadvantage of the normal GaAs 18 mil diameter homostructure hemispherical emitter is that the increased path length leads to increased absorption and thus the lower values of radiance. GaAlAs 18-mil diameter hemispherical emitters with graded AlAs composition to reduce absorption have been fabricated in the past. However, very thick GaAlAs epitaxial layers are required (9-10 mils). A technique for the growth of very thick GaAlAs epitaxial layers has been developed but the technique is not reproducible and is not compatible with multi-layer double heterostructure junctions which have high radiances.
One approach to eliminate the requirement for thick transparent GaAlAs epitaxial layers is to greatly reduce the diameter of the hemisphere. To do this directly is not practical because of the difficulty of handling emitters with diameters less than 10-15 mils in diameter and because of a need of a minimum spacing between the N and P contacts on the base of the hemisphere to prevent formation of shorts during solder mounting of the devices on silicon submounts.
A more practical approach for achieving smaller diameter shaped emitting surfaces is to use an etching process to form an integral lens in a larger size device chip. The process for forming the shaped emitting surface is part of the new invention.
A previous approach for an integral lens structure has been described in the article "Integral Lens Coupled LED" by F. D. King and A. J. Springthorp, published in Electronic Materials Number 4, Page 243, 1975. The approach was to etch an array of small hemispherical holes in the GaAs substrate before growth of a multi-layer GaAlAs structure. The epitaxial growth fills in the etched holes. During device processing, a preferential etch (etches GaAs but not GaAlAs) was used to remove localized regions of the GaAs substrate from the backside of the slice and to leave a GaAlAs microlens structure. However, only a slight increase in optical output power and device performance was achieved. The limitation of this approach is that the hemispherical lenses are small compared to the junction size, because of the difficulty of filling in very deep holes by liquid phase epitaxial techniques. For optimum improvement with a hemispherical shape, the junction should be small compared to the lens size. This can be accomplished by the process described in this invention.